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		<title>three.js webgl - materials - cubemap mipmaps</title>
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			<a href="https://threejs.org" target="_blank" rel="noopener">three.js</a> - rendering to cubemap mip levels demo.<br/>
			Left: original cubemap<br/>
			Right: generated cubemap<br/>
		</div>

		<script type="importmap">
			{
				"imports": {
					"three": "../build/three.module.js",
					"three/addons/": "./jsm/"
				}
			}
		</script>

		<script type="module">

			import * as THREE from 'three';
			import { OrbitControls } from 'three/addons/controls/OrbitControls.js';

			let container;
			let camera, scene, renderer;

			const CubemapFilterShader = {

				name: 'CubemapFilterShader',

				uniforms: {
					cubeTexture: { value: null },
					mipIndex: { value: 0 },
				},

				vertexShader: /* glsl */ `

					varying vec3 vWorldDirection;

					#include <common>

					void main() {
						vWorldDirection = transformDirection(position, modelMatrix);
						#include <begin_vertex>
						#include <project_vertex>
						gl_Position.z = gl_Position.w; // set z to camera.far
					}

					`,

				fragmentShader: /* glsl */ `

					uniform samplerCube cubeTexture;
					varying vec3 vWorldDirection;

					uniform float mipIndex;

					#include <common>

					void main() {
						vec3 cubeCoordinates = normalize(vWorldDirection);

						// Colorize mip levels
						vec4 color = vec4(1.0, 0.0, 0.0, 1.0);
						if (mipIndex == 0.0) color.rgb = vec3(1.0, 1.0, 1.0);
						else if (mipIndex == 1.0) color.rgb = vec3(0.0, 0.0, 1.0);
						else if (mipIndex == 2.0) color.rgb = vec3(0.0, 1.0, 1.0);
						else if (mipIndex == 3.0) color.rgb = vec3(0.0, 1.0, 0.0);
						else if (mipIndex == 4.0) color.rgb = vec3(1.0, 1.0, 0.0);

						gl_FragColor = textureCube(cubeTexture, cubeCoordinates, 0.0) * color;
					}

					`,
			};


			init();

			async function loadCubeTexture( urls ) {

				return new Promise( function ( resolve ) {

					new THREE.CubeTextureLoader().load( urls, function ( cubeTexture ) {

						resolve( cubeTexture );

					} );


				} );

			}

			function allocateCubemapRenderTarget( cubeMapSize ) {

				const params = {
					magFilter: THREE.LinearFilter,
					minFilter: THREE.LinearMipMapLinearFilter,
					generateMipmaps: false,
					type: THREE.HalfFloatType,
					format: THREE.RGBAFormat,
					colorSpace: THREE.LinearSRGBColorSpace,
					depthBuffer: false,
				};

				const rt = new THREE.WebGLCubeRenderTarget( cubeMapSize, params );

				const mipLevels = Math.log( cubeMapSize ) * Math.LOG2E + 1.0;
				for ( let i = 0; i < mipLevels; i ++ ) rt.texture.mipmaps.push( {} );

				rt.texture.mapping = THREE.CubeReflectionMapping;
				return rt;

			}

			function renderToCubeTexture( cubeMapRenderTarget, sourceCubeTexture ) {

				const geometry = new THREE.BoxGeometry( 5, 5, 5 );

				const material = new THREE.ShaderMaterial( {
					name: CubemapFilterShader.name,
					uniforms: THREE.UniformsUtils.clone( CubemapFilterShader.uniforms ),
					vertexShader: CubemapFilterShader.vertexShader,
					fragmentShader: CubemapFilterShader.fragmentShader,
					side: THREE.BackSide,
					blending: THREE.NoBlending,
				} );

				material.uniforms.cubeTexture.value = sourceCubeTexture;

				const mesh = new THREE.Mesh( geometry, material );
				const cubeCamera = new THREE.CubeCamera( 1, 10, cubeMapRenderTarget );
				const mipmapCount = Math.floor( Math.log2( Math.max( cubeMapRenderTarget.width, cubeMapRenderTarget.height ) ) );

				for ( let mipmap = 0; mipmap < mipmapCount; mipmap ++ ) {

					material.uniforms.mipIndex.value = mipmap;
					material.needsUpdate = true;

					cubeMapRenderTarget.viewport.set( 0, 0, cubeMapRenderTarget.width >> mipmap, cubeMapRenderTarget.height >> mipmap );

					cubeCamera.activeMipmapLevel = mipmap;
					cubeCamera.update( renderer, mesh );

				}

				mesh.geometry.dispose();
				mesh.material.dispose();

			}

			function init() {

				container = document.createElement( 'div' );
				document.body.appendChild( container );

				// Create renderer
				renderer = new THREE.WebGLRenderer( { antialias: true } );
				renderer.setPixelRatio( window.devicePixelRatio );
				renderer.setSize( window.innerWidth, window.innerHeight );
				renderer.setAnimationLoop( animate );
				container.appendChild( renderer.domElement );

				scene = new THREE.Scene();

				camera = new THREE.PerspectiveCamera( 50, window.innerWidth / window.innerHeight, 1, 10000 );
				camera.position.z = 500;

				// Create controls
				const controls = new OrbitControls( camera, renderer.domElement );
				controls.minPolarAngle = Math.PI / 4;
				controls.maxPolarAngle = Math.PI / 1.5;

				window.addEventListener( 'resize', onWindowResize );

				// Load a cube texture
				const r = 'textures/cube/Park3Med/';
				const urls = [
					r + 'px.jpg', r + 'nx.jpg',
					r + 'py.jpg', r + 'ny.jpg',
					r + 'pz.jpg', r + 'nz.jpg'
				];

				loadCubeTexture( urls ).then( ( cubeTexture ) => {

					// Allocate a cube map render target
					const cubeMapRenderTarget = allocateCubemapRenderTarget( 512 );

					// Render to all the mip levels of cubeMapRenderTarget
					renderToCubeTexture( cubeMapRenderTarget, cubeTexture );

					// Create geometry
					const sphere = new THREE.SphereGeometry( 100, 128, 128 );
					let material = new THREE.MeshBasicMaterial( { color: 0xffffff, envMap: cubeTexture } );

					let mesh = new THREE.Mesh( sphere, material );
					mesh.position.set( - 100, 0, 0 );
					scene.add( mesh );

					material = material.clone();
					material.envMap = cubeMapRenderTarget.texture;

					mesh = new THREE.Mesh( sphere, material );
					mesh.position.set( 100, 0, 0 );
					scene.add( mesh );

				} );

			}

			function onWindowResize() {

				camera.aspect = window.innerWidth / window.innerHeight;
				camera.updateProjectionMatrix();

				renderer.setSize( window.innerWidth, window.innerHeight );

			}

			function animate() {

				renderer.render( scene, camera );

			}

		</script>

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